Ideally, it should only occur when you tap and release Alt, not when you use Alt as a modifier (hold Alt and press another key). Ideally, but there are a lot of window managers and it doesn't always work as expected.

Problem occurs when you press Tab-key after pressing Alt-key. Only Opera has this kind of issue and I don't want to remap Alt-Tab just for Opera. I am trying to use Super+Tab but I am not really comfortable with it too.